Ellipsometry is an optical technique used to investigate properties of a specimen sample. Generally, ellipsometry may be defined as the measurement of the state of polarized light waves. An ellipsometer apparatus is known in the art and measures the changes in the polarization state of light when it interacts with a specimen sample. The ellipsometer apparatus is used in many different areas including physics, chemistry, materials science, biology, mechanical engineering, metallurgical engineering and biomedical engineering. One particular area where ellipsometers are used is in investigating properties of thin films on a specimen sample such as by measuring a thickness of a thin film formed on a surface of the specimen sample. Other properties can also be determined such as volume, chemical composition, roughness, index of refraction and electrical conductivity.
One specific application of ellipsometers is in evaluating the physical properties of fuels, such as in thermal oxidation testing of jet fuels. Jet fuels are subjected to thermal stress in the operation of an aircraft due to designs that utilize jet fuel as a coolant in various heat exchangers used in jet engine systems. These thermal stresses can affect the performance of the fuel. Fuel manufacturers continue in their efforts to develop more thermally stable jet fuels as well as enhance their ability to better test the thermal stability of jet fuels.
The American Society for Testing and Materials (ASTM) developed a test, commonly referred to as The Jet Fuel Thermal Oxidation Test that is widely used to determine the thermal stability of jet fuels. The test, as designated by ASTM D3241, is the standardized test procedure used to assess the thermal stability of conventional jet fuels. As part of this test, jet fuel is first passed over a specimen sample in the form of a heated metal tube. The thermal stability of the fuel is characterized by the amount of deposits adhering to the tube. The deposits form a thin film and thus, the thickness of the thin film is an indication of the thermal stability of the fuel. The ellipsometer apparatus is then used to determine the film thickness and determine the thermal stability of the jet fuel.
When preparing the tube sample to be analyzed in the ellipsometer, the tube can sag, bend or warp or otherwise become distorted. The tube is typically formed from aluminum or stainless steel and the heat applied to the tube in conjunction with the fuel being passed over the tube sometimes results in bending of the tube. Such bends or distortions can affect the ellipsometer analysis and consequently the accuracy of the measurements taken by the ellipsometer. For example, a most accurate film thickness measurement is determined by the ellipsometer at the bottom center of the specimen tube. Thus, the beam of light from the light source assembly is directed to the bottom center of the specimen tube. If the specimen tube is bent, warped etc., the beam of light will not be directed at the actual true bottom center of the specimen tube. Accordingly, film thickness measurements will be off or inaccurate. Because the ellipsometer is capable of measuring down to extremely minute thicknesses, even a slight bend in the tube can significantly affect the accuracy of measurements taken by the ellipsometer. Efforts have been made to straighten the tube samples prior to mounting the tubes in the ellipsometer. These straightening techniques, however, have led to other problems such as broken tubes or discontinuity of the thin film being measured. Current ellipsometers do not have any mechanisms to compensate for such tube shape distortions or deviations.
While such ellipsometers according to the prior art provide a number of advantageous features, they nevertheless have certain limitations. The present invention is provided to overcome certain of these limitations and other drawbacks of the prior art, and to provide new features not heretofore available. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.